The present invention relates to a tire for motorcycles, and more particularly to a radial tire for motorcycles which is improved in preventing cracks from developing at the bottoms of longitudinal grooves of the tire tread.
With recent prevalence of pavement, it has been intended to raise the speed of vehicles and, therefore, motorcycle tires are also required to possess characteristics capable of accommodating the high speed running.
Generally, for the carcass of motorcycle tire, widely employed is a cross-ply structure, that is, such a structure that ply cords of the carcass intersect each other at an angle of about 30.degree. to 60.degree. with respect to the circumferential direction of the tire. The reason is that the tire for motorcycle is basically different from that for four-wheel vehicles such as passenger car, particularly, in motional functions at the time of turning, and is required to have characteristics to maintain stable running while resisting centrifugal force acting upon the motorcycle body with a force (camber thrust) generated when the motorcycle is tilted during turning at a large angle to a plane perpendicular to the road surface (when a large degree of camber angle is given), which horizontally acts along the direction in which the camber angle is given. Therefore, the motorcycle tire is strengthened in transverse stiffness by employing a carcass of the cross-ply structure as described above so as to maintain the camber thrust. The radial structure inferior in transverse stiffness has been scarcely adopted for the motorcycle tires from the above-mentioned point of view. However, the tire of the cross-ply structure has the drawbacks that transverse vibration (weave phenomenon) of the motorcycle body resulting from cornering power and transverse stiffness of the tire occurs during high speed running and that the abrasion resistance is low. As countermeasures to such problems, the cord angle of carcass to the circumferential direction of the tire is lowered or the number of carcass plies is increased for increasing the tire stiffness, but the drawbacks attributable to the structure cannot completely be removed as far as the cross-ply structure is employed.
Although radial tires have the drawback of poor transverse stiffness, they are superior in steering stability in high speed running and abrasion resistance. It is known to dispose a breaker to reinforce the tires, whereby the tread stiffness is raised and the abrasion resistance and steering stability are improved. On the other hand, in case of the motorcycle tires, it causes a problem that the tread surface becomes susceptible to deformation stress in the radial direction because of a large curvature in the tread section and a small thickness of the tread and, consequently, cracking is easy to occur at the bottoms of the tread grooves. For the reasons as mentioned above, the radial structure has been scarcely applied to tires for motorcycles.